1. Field of the Invention
Embodiments of the invention generally relate to optical sensing systems and, more particularly, to compensating for spectral fluctuations in broadband source light used to interrogate optical sensors.
2. Description of the Related Art
Most optical sensors include an optical fiber with a sensitized region exposed to an environmental condition. The sensitized region modulates an input or interrogating light signal transmitted within the optical fiber. This modulation can alter one or more parameters of the interrogating light transmitted within the optical fiber, such as amplitude, power distribution versus frequency/wavelength, phase or polarization. Analysis of modulated response light emerging from the fiber determines values indicative of the environmental condition. Such fiber-optic sensing techniques utilize sensors based on, for example, Bragg gratings or interferometers, to measure a wide variety of parameters, such as strain, displacement, velocity, acceleration, flow, corrosion, chemical composition, temperature or pressure.
As an example of these optical sensors, a Bragg grating sensor operates by modulating the wavelength of an input light field transmitted through an optical waveguide. The Bragg grating sensor comprises a tuned optical filter, or “Bragg grating,” imprinted upon the core of an optical waveguide coupled to a broadband light source. The Bragg grating is structured to reflect light within a narrow bandwidth centered at a Bragg wavelength corresponding to the spacing of the Bragg grating. If the Bragg grating sensor is strained, for example by stress or vibration, the Bragg grating spacing changes. This change in spacing results in a shift in the reflected light wavelength, which can be measured and correlated with the magnitude of the stress or vibration.
Instabilities in a center wavelength of input light provided by a broadband light source may cause variations in sensor response signals produced upon the interrogating light arriving at the optical sensor. For example, broadband sources producing input light without a stable center wavelength when used with a Bragg grating sensor may cause variations in the reflected response signal emitted by the sensor, resulting in incorrect measurements or undesirable noise. Accurate and reliable measurements determined by detection of response signals from the optical sensors require a broadband light source outputting light with a center wavelength that does not drift around with time or other environmental changes. However, attempts in many environments to achieve such a stable broadband light source by stabilization and control (e.g., temperature stabilization or vibration dampening) of components proves difficult, expensive and oftentimes insufficient.
Therefore, there exists a need for improved optical sensing configurations and methods that compensate for center wavelength fluctuations in light produced by a broadband source used to interrogate optical sensors.